1. Field of the Invention
This invention relates to an improved apparatus for the plating of material, and more particularly to the vacuum plating of crystals in a circuit board.
2. Description of the Prior Art
Crystals can be made to oscillate at a natural frequency, or by appropriate and selective plating, can be manufactured to oscillate at a desired exact frequency. With the increased importance of electronics in our daily lives, crystals have gained widespread use, particularly in the form of circuit boards, often used as clock oscillators.
Depending upon how exacting the frequency is to be set, for plating of clock oscillator crystals, the user will base coat the crystal to a rough frequency, mount it on an oscillator circuit board, and finally callibrate the completed circuit by means of plating additional material on the crystal to tune the entire circuit as a whole.
In other plating operations, individual crystals can be plated in the apparatus. In those cases the crystal may be first base plated on one side, or both sides before final plating, again on one or both sides. In the more accurate frequency work it is recommended to coat both sides of the crystal to avoid "spurs" which is the occurrance of random peaks caused by aberrations of unknown origin.
In the frequency range of the present invention, the fundamentals and overtones thereof have been tested to 200 Mega Hertz (MHz) and above and have tested out with great accuracy of 2 ppm up to frequencies of at least 180 MHz.
Prior to the invention of U.S. Pat. Nos. 4,236,487 and 4,323,031 to Kaplan, of which this invention is an improvement thereon vacuum crystal plating was done in a bell jar in which the crystal had to be internally loaded with tweezers into a special holder within the bell jar. The entire bell jar was evacuated including the special holder for the crystal. The vacuum was drawn through a narrow opening in the jar. The vacuum was not as absolute as desired. The relative size of the vacuum equipment, the complexly shaped items that were within the vacuum chamber, and multiple insulated wires disposed inside the vacuum chamber, provided multiple surfaces to which air or other contaminants could adhere. The presence of these contaminants, together with the difficulties in removing them resulted in deficiencies in the plating process and resulted in deficiencies in the product produced.
Additionally, the internal loading of the crystals within the bell jar was relatively time consuming and, therefore, expensive. Also, because of the size and shape of the vacuum equipment, much of the noble metal vaporized in the plating process never reached the crystals to be coated. Instead the metal was randomly deposited on the walls of the jar or surfaces of the crystal holder. Therefore, the plating process was lengthened and much of the metal used in the plating process was ineffectively used.
Additionally, the random depositing of conductive material within the chamber produced shorting or other electrical conduction problems. Further, the random depositing of the precious metals on the vacuum sealing surfaces such as "O" rings and mating surfaces caused deterioration of these surfaces and therefore, resulted in vacuum leaks.
The randomly deposited noble metal was recovered by cleaning the surfaces within the bell jar. However, because of the complexly shaped and relatively large surfaces the cleaning process was not as quick or easy as desired. Such apparatus as these are exemplified by U.S. Pat. No. 2,765,765 to Bigler et al, U.S. Pat. No. 3,756,851 to Rennick et al, U.S. Pat. No. 3,864,161 to Thompson, and and U.S. Pat. No. 4,112,134 to Buynak et al.
To overcome most of these prior art problems, the aforementioned invention of Kaplan was produced. Kaplan's device provided a crystal plating device which reduced the vacuum pumping time due to its relatively small vacuum chamber, enabled alignment of the crystal to be plated to be made externally of the vacuum chamber thus reducing the extent of alignment of the crystal within the vacuum chamber, provided a plating device which was relatively easy to clean, minimized the amount of time required for loading of the chamber, facilitated effective use of the noble metal, and had many numerous other advantages.
Often, however, it is desirable to plate crystals after they have been placed in a circuit on a circuit board. The aforementioned Kaplan's described device was incapable of accomplishing this without modifications that were relatively impractical. Additionally, filaments have been developed which were essentially a conductive filament strip having a plurality of areas of noble metal attached to the strip. The filament holder described in Kaplan was cumbersome to use with such type filament.